The present invention generally relates to methods and devices for cooling fluids, and more particularly to the construction and configuration of fire tube boilers and water tube boilers, and still more particularly to increasing the durability of cooling tubes used in such devices.
Many industrial processes employ high temperature heat exchangers, sometimes called fire tube boilers or water tube boilers, to remove heat from a fluid stream, either a gas or a liquid. The construction of these fire tube boilers has been a source of interest for many engineers, since the high temperatures typically experienced in these processes result in many equipment problems, such as corrosion, deterioration of materials by cracking, compromise of junctions between dissimilar materials, uneven expansion of materials in the equipment, and the like. Illustrative of these problems are processes that involve the removal of hydrogen sulfide gas from certain industrial processes.
Many industrial processes result in the production of hydrogen sulfide (H2S), an odorous, corrosive, and highly toxic gas. Hydrogen sulfide is generally undesirable because of these qualities and also because it deactivates industrial catalysts. H2S is also commonly found in natural gas and at oil refineries, especially if the crude oil contains a lot of sulfur compounds. Because H2S is such an undesirable substance in these applications, industrial processes may typically include provisions to convert H2S to other non-toxic and less corrosive substances. One such method of converting H2S to elemental sulfur is well known in the art as the Claus Sulfur Recovery process.
After the H2S is separated from a host gas stream as, for example, by using amine extraction, it is fed to an apparatus supporting the Claus Sulfur Recovery Process, where it is converted in two separate steps. The first step involves partially oxidizing the H2S with ⅓ of the necessary oxygen in a reaction furnace at high temperatures, typically 1000° C.-1400° C. Sulfur is formed thereby, but the resulting gas comprises about ⅔ H2S and about ⅓ SO2. This resulting gas is then passed through a water-cooled heat exchanger known in the art as a fire tube boiler, to remove some of the heat from the resulting gas. The second step involves reacting the remaining H2S and SO2 at lower temperatures (about 200-350° C.) over a catalyst to make more sulfur. A catalyst is needed in the second step to help the components react with reasonable speed, but unfortunately the reaction does not go to completion even with the best catalyst. Thus two or three stages are used, with sulfur being removed between the stages, and multiple stages of the process may employ multiple fire tube boilers.
In a fire tube boiler used in the first step, the hot gasses pass directly through tubes suspended within a vessel containing water as the cooling medium. Fire tube boilers may be designed for vertical, inclined or horizontal orientations, with the preferred position being horizontal. A number of such tubes may be attached to tube sheets that make up the ends of a cylindrical vessel, so that the tubes are suspended within the cooling medium without touching one another. This structure allows the cooling medium to pass around and between the tubes, so that heat is transferred from the fluid passing through the tubes through the tube walls to the cooling medium by means of conduction and convection.
The high temperatures encountered in such applications, illustrated by the Claus process for example, have resulted in a number of problems for fire tube boilers. First, when the high temperature gas enters the relatively cool interior of a cooling tube, a phenomenon known as film boiling may occur on the exterior of the tube. In film boiling, the exterior surface of the cooling tube is heated rapidly, and a layer of steam is generated around the cooling tube. Thus, the water that would otherwise surround the tube is prevented from contacting the tube by the resultant steam layer so that the cooling water is not in direct contact with the exterior surface. At the high temperatures exhibited by the Claus process, for example, the water along this portion of the tube surface can vaporize and form a steam layer preventing the liquid water from contact with the tube. As a result, heat transfer from the tube exterior surface to the water occurs mainly through radiation, which is less efficient than conduction. Thus, film boiling along tube surface reduces the efficiency of the fire tube boiler and can increase the temperature of the tube wall to a damaging level.
A second common problem with many heat exchanging devices, such as fire tube boilers, is erosion, usually caused by the velocity of flow of the high temperature gas especially adjacent the ends of the tube and over the first few centimeters inside of the tube where the fluid flow may be turbulent. This problem may be exacerbated by the presence of foreign materials that may be entrained within the gas flow, such as soot or ash in some applications. Erosion necessitates the replacement of tubes, so that if erosion could be reduced, then the frequency of replacement would be reduced.
A third common problem is corrosion that can be caused by reaction of the gas with the interior surfaces of the tubes. When the fluid being cooled is H2S, the formation of scale composed of iron sulfide has been observed on the interior tube walls. This problem may be solved by choosing tube materials that are non-reactive with the incoming gas, but other considerations such as resistance to high temperatures may outweigh the need for reduced corrosion. Furthermore, the junction between the tube and the tube sheet may be vulnerable to such corrosion problems.
A fourth problem that is closely associated with that of corrosion is the exhaustion of ductility of the tube material resulting from extreme swings of temperature within a very short distance. This repeated heating and cooling may result in cyclic strain accumulation of the tube structure or of the connective structure between the tube and the tube sheet, resulting in cracking, metal fatigue, or other types of damage.
The prior art is replete with examples of how the problem of attaching a tube end to a tube sheet is addressed, when used in the application of heat exchangers, boilers, flues, and the like. For example, U.S. Pat. No. 1,102,163, to Opperud, discloses an attachment method, wherein the tube end is inserted through the tube sheet and internally expanded to form a lip engaging the tube sheet and an opening with a slightly expanded portion sufficient to receive a cylindrical thimble inserted within the slightly expanded portion. A ring is then the tube sheet snugly between it and the lip. The method requires precise placement of the ring and expansion of the tube end to precisely capture the tube sheet. Care must be taken to ensure that the joint is snug so that the pressure of internal water does not seep through the opening between the tube and the tube sheet.
U.S. Pat. No. 3,317,222, to Maretzo, discloses insert constructions for tubes of heat exchangers that protect from deterioration the tube interiors and tube end portions of said tubes, as well as the regions where the tube end portions are welded to the tube sheet. The invention consists of a tube insert with a flared end, with the tube insert being inserted into a tube end that is welded in a hole of the tube sheet. The tube end has a circumferentially expanded portion that abuts the interior walls of a hole for a snug fit in the hole. The tube insert is inserted into the tube end, the flared end protecting the weld, so that a tapered end of the tube insert extends and tapers a distance into the tube. The portion of the tube insert adjacent to the expanded portion of the tube end is then expanded to form a pressure fitting against the interior of the tube end to hold the tube insert in place. However, the expanded portion of the tube insert presents ridges to an incoming flow of gas, which may cause unwanted turbulence in the gas stream and possible wear.
As can be seen, there is a need for a method for attachment of a tube end to a tube sheet in a fire tube boiler, which prevents or reduces film boiling, corrosion, and fatigue of the tube end. Furthermore, turbulence of the entering gas should also be reduced to promote improvement of the service life of the tube.